当我们运行下面提到的代码下载并保存在照片库中的图像，它在iOS 11中自动崩溃。请指导解决这个问题。

try 
{
    string uri = "https://www.xamarin.com/content/images/pages/branding/assets/xamagon.png";
    using (var url = new NSUrl(uri))
    using (var data = NSData.FromUrl(url))
    UIImage.LoadFromData(data).SaveToPhotosAlbum((image, error) => 
    {
        var o = image as UIImage;
        Console.WriteLine("error:" + error);
    });
} 
catch(Exception exx) 
{
    throw exx;
}